Porous polytetrafluoroethylene and a process for the production thereof

ABSTRACT

A new porous polytetrafluoroethylene (PTFE) and a process for the production thereof is provided. Both the PTFE and the process excel by the fact that polytetrafluoroethylene is radiation-chemically degraded, the degraded polytetrafluoroethylene is mixed with a high-molecular emulsion polymerisate of polytetrafluoroethylene and the mixture is extruded, stretched and may additionally be sintered.

This application is a division of application Ser. No. 08/214,939, filedMar. 16, 1994, allowed.

FIELD OF THE INVENTION

The invention relates to a modified porous polytetrafluoroethylene and aprocess for its production.

BACKGROUND OF THE INVENTION

Microporous polytetrafluoroethylene and the production thereof was firstdescribed in U.S. Pat. Nos. 3,953,566 and 4,187,390. The materialconsists solely of high-molecular emulsion polymerisate of PTFE(hereinafter referred to as PTFE) which is paste-extruded and thenexpanded. U.S. Pat. No. 3,953,566 describes the production of anexpanded microporous PTFE with a stable structure, in which coagulateddispersions are used as starting materials. These products have found awide range of applications due to their chemical and physical stability.

The state of the art also includes modifications of PTFE having certainchemical and physical properties. U.S. Pat. No. 5,098,625, for instance,describes a process for the production of porous PTFE-membranes ofincreased density, improved flexibility and reduced cold flow which arealso made from coagulated dispersions. This type of PTFE, however, hasless tensile strength than the unmodified PTFE.

EP 418155 proposes to partially sinter high-molecular PTFE and expand itafterwards to improve thermal stability and dimensional stability and toincrease porosity.

Furthermore U.S. Pat. No. 5,064,593 describes a multilayer PTFE-membranewhich comprises a small-pored filter layer made from a non-fibrilforming PTFE fine powder and an open-pored supporting layer made fromhigh-molecular PTFE. Each layer must be produced separately and theseparate layers are linked with each other during processing.

U.S. Pat. No. 5,102,921 describes a PTFE-material of high porosity witha large pore diameter, which is obtained by a mixture of ahigh-molecular and a low-molecular PTFE fine powder. These products,however, have a low mechanical strength.

In this connection, U.S. Pat. No. 5,087,641 suggests a process in whicha microporous membrane is subsequently modified with a dispersion of asintered or irradiated PTFE material. The additional component isincorporated into the pore interstices, which creates a certainreinforcing effect.

A modification of the properties of porous PTFE material by admixturesof low-molecular PTFE is desirable in particular with a view to thereutilization of polymerisate scrap.

Generally speaking high-molecular emulsion polymerisates of PTFE canform fibril structures during the paste extrusion and expansion steps.Low-molecular PTFE-types, however, cannot be expanded and thus will notform any fibril structures.

Experts know that irradiated PTFE generally have reduced mechanicalstrength.

There is a need for a material of porous polytetrafluoroethylene whichcan be produced by incorporating perfluorinated non-expandable polymersinto high-molecular PTFE-material without compromising the desirablemechanical properties such as tear resistance, tensile strength andbreak elongation.

SUMMARY OF THE INVENTION

A porous polytetrafluoroethylene material comprising a mixture ofdegraded polytetrafluoroethylene and a high molecular weightpolymerisate of polytetrafluoroethylene is provided wherein the mixtureis further subjected to a process comprising extrusion and expansionsteps. The porous polytetrafluoroethylene material may also be sintered.The degraded polytetrafluoroethylene may be degraded by a radiationdegradation process by means of electron rays. The energy dose may rangefrom 10 to 3000 kGy. The degraded polytetrafluoroethylene has amolecular weight of at most 10⁶. The emulsion polymerisate has anaverage molecular weight of between 2×10⁶ and 50×10⁶. The amount ofdegraded polytetrafluoroethylene relative to the total mass of thematerial is between 1 and 50%. The degraded polytetrafluoroethylene hasa particle size from 0.1 to 100 micrometers.

A process for the production of porous polytetrafluoroethylene materialis also provided comprising the steps of a) degradingpolytetrafluoroethylene by radiation; b) mixing the degradedpolytetrafluoroethylene with a high molecular emulsion polymerisate ofpolytetrafluoroethylene to form a mixture; c) extruding; and (d)expanding to form a porous material. The porous material may also besintered.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photomicrograph of a cross-section of the membrane preparedin accordance with Example 4 at 2000×magnification.

FIG. 2 is a graphical printout of the Fourier Transformation infraredspectrophotometer adsorption scan of the microporous PTFE structure madein accordance with Example 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A modified porous polytetrafluoroethylene is provided comprising amixture of PTFE that has been degraded by exposure to radiation and ahigh-molecular emulsion polymerisate of PTFE. The mixture is thenextruded, expanded and optionally sintered (heated above the crystallinemelt point of PTFE). A method of making this modifiedpolytetrafluoroethylene is also provided.

Surprisingly, this modified porous polytetrafluoroethylene has excellenttear elongation and tear resistance value. Tensile strength and tearresistance are higher than that of pure high-molecular microporous PTFE.This was not expected with the use of radiation degraded PTFE. Loadingand coating experiments with the expanded compounds have shown that themodified polytetrafluoroethylene performs better than conventionalmicroporous PTFE such as those made in accordance with the methoddescribed in U.S. Pat. No. 3,953,566.

Both unsintered and sintered PTFE powder as well as microporous PTFE(with no additives) which is PTFE that has been expanded and sintered(such as those products sold by W. L. Gore & Associates, Inc.) may besubjected to a radiation degradation process and further used in theinventive process. The emulsion polymerisate may be a homo- orcopolymerisate of PTFE. The strength of the resulting mixed polymerisateis dependent on the average particle size of the radiation degradedPTFE, its chemical composition and starting crystallinity, all of whichcan be influenced by the type of radiation utilized. The porosity andthe average pore size of the resulting membranes is dependent on theamount of radiation degraded component used, the content of lubricant,the mixing procedure and the extrusion pressure.

The term "high-molecular PTFE-emulsion polymerisate" denotespolymerisates of PTFE with an numerical average molecular weight M_(n)of 2×10⁶ to 50×10⁶. In general, these types are used for the productionof expanded porous membranes. The radiation degraded perfluorinatedpolymer usually has numerical average molecular weight M_(n) of lessthan 10⁶. Such polymerisates are not expandable.

The radiation degradation is effected with energy rich radiation,preferably with the use of electron rays. The radiation can be effectedin the presence of oxygen from the air or under an inert gas atmosphere(such as N₂). An emulsion or suspension polymerisate of PTFE orPTFE-scrap can be used. The degradation products are preferablysubjected to tempering at a temperature above 150° C. in order to removeradicals, low-molecular fission products and hydrogen fluoride. Anenergy dose ranging from 10 to 3000 kGy has turned out to beparticularly suitable to obtain maximum crystallinity and dense molarmass distributions in the PTFE. A preferred energy dose ranges from 50kGy to 800 kGy. Gy is an SI unit and is equivalent to joule/kg.

Scrap of all known types of PTFE are suitable for radiation degradation.The starting material of scrap PTFE is preferably subjected to acleaning procedure and pre-radiation to make the material brittle. Theradiation degradation of the cleaned scrap PTFE then continues similarto the process used on pure PTFE-materials. The radiation results in theembrittlement of the PTFE-material, which facilitates the grinding ofthe material. Thus a very low particle size PTFE material is obtained.The molecular weight of the PTFE-material is considerably lowered andits crystallinity considerably increased. The content of thenon-expandable, radiation degraded component which is added to thehigh-molecular PTFE together with the lubricant is 1 to 50% by mass,relative to the total PTFE mass and is preferably 4 to 30% by mass. Ifthe content of the degraded component is more than 50%, problems may beencountered during processing.

The particle size of the radiation-degraded material ranges from 0.1 to100 micrometers, preferably 0.2 to 20 micrometers. Larger particleschange the surface structure of the product and initiate defects in thenode/fibril areas.

Suitable lubricants comprise a wide variety of liquids which arenormally used for paste extrusion. Such lubricants include diesel oils,hydrocarbons, toluol, silicone oils, fluorocarbons, polymer solutions,mixtures thereof and water or aqueous surfactant solutions. For thetests described in the following examples, an i-paraffin oil with aboiling range of 181° C. to 212° C. was used. The lubricant content,relative to the PTFE-mixture, depends on the type and polymeric chainlength of the added degraded component and the desired extrusionpressure. Usually the amount of lubricant added is 10 to 50% by mass,relative to the mixture and is preferably 20 to 40% by mass.

The mixing process of the high-molecular PTFE emulsion polymerisate withthe radiation-degraded component and the lubricant is variable. Forinstance, the high-molecular PTFE may be pre-mixed in a dry mixer withthe radiation-degraded component, after a certain time the lubricant isadded and a mixing process ensues afterwards. The mixing time isvariable and mainly depends on the type of the component which has beendegraded by radiation. Preferably the mixing time is between 10 minutesand 180 minutes. Mixing is continued until a complete homogenizationresults.

Alternatively, the radiation degraded component may first be added tothe lubricant and mixed to create a homogenous dispersion which is thenadded to the PTFE emulsion polymerisate. Another alternative includesproducing a homogenous micro-powder disperison of the radiation-degradedcomponent which is then added to a PTFE-dispersion. Subsequently, thedispersion is precipitated, (e.g. with a polyelectrolyte) water isremoved, the material is dried and a lubricant is then added to themixture.

Subsequent steps including the production of the pressed billet, thepaste extrusion with the subsequent calendering step and the removal ofthe lubricant, the stretching or expansion and the sintering process aredescribed in detail in German Patent 2417901 and U.S. Pat. No.3,962,153. These patents are hereby incorporated by reference. Uniaxialor biaxial expansion may be applied. Expansion rate is defined as theamount of expansion divided by the amount of time elapsed duringexpansion.

The radiation degraded perfluorinated polymerisate has a low molecularweight and a lower melting point than the expanded porous PTFE whichforms the predominant structure of the resulting product. At highertemperatures, the lower chained molecules are more movable and tend tomigrate to the surface of the nodes. The lower chained molecules arere-oriented in the highly crystalline range and are subject to an orderinfluenced in the non-melted polymer part of the fibril structure. As aresult, with the end groups of the added short-chained polymerisatesgrouped on the node surface, the DSC (differential scanningcalorimetrics=differential thermo-analysis) shows a broader main meltingpoint, and the melting heat/recrystallization heat of the compoundmaterials is higher compared to materials made from pure high-molecularPTFE such as those described in U.S. Pat. No. 4,187,390. It appears thathighly ordered micro-crystallite structures are created causing theresulting material to have greater strength.

The increased number of end groups in the expanded material and theaddition of electron-irradiated PTFE additionally changes the surfaceproperties of the expanded PTFE. Materials including membranes areproduced with an improved affinity to adhesive agents and with ionic andpolar bonds. This is highly desirable because it simplifies theadmixability of fillers, the metallisation of such materials and thedurability of coating systems, for example, with polymer coatings onexpanded PTFE. The resulting product may be in any shaped article suchas membrane fiber or rod.

The following examples more clearly illustrate the process of making themodified product and resulting properties. Chemical and physical testmethods used i n the examples include:

Molecular Weight

For low molecular weight PTFE that has been degraded, the molecularweight is determined by the formula: ##STR1## wherein Tm=meltingtemperature in °K.

Higher molecular weights of PTFE may be determined from the formula(Doban & Sperati): ##EQU1##

Particle Size

The data for the particle sizes of commercially available polymers weretaken from data sheets of the manufacturer. Supplementary measurementswere made with a particle measuring unit from the company Leeds &Northrup Microtrac, FRA Model 9250, Particle Size Analyzer.Approximately 0.5 g of pulverized PTFE was dispersed in 20 ml of octane.The mixture was homogenized in an ultrasonic bath, placed in a flowthrough cell of the particle size analyzer. The particle size wasdetermined by laser bending.

Average Pore Diameter/Bubble Point

A sample membrane having a 25 mm diameter was obtained and wetted withperfluoroether. The wetted sample was placed in a Coulter Porometerwherein the average pore diameter of the final product was determined.

The bubble point was determined by measuring the pressure at which airbubbles permeate a sample that has been wetted with isopropanol. Asample membrane that had been wetted with isopropanol was placed on aporous carrier covered with a woven carrier and clamped into place. Airpressure was applied from below and the pressure increased at discreteincrements. The pressure was recorded at the instance at which the firstbubble was formed.

Gurley Number

A sample was placed on a porous carrier forming the base of a column.The top plate of the column was pressed down at a pressure of 1.2 kPa,and the time was determined at which 100 cm³ passed through an area of6.45 cm³ to yield air permeability.

Differential Scanning Calorimetry (DSC)

A 10 mg sample was placed in an oven and heated at a rate of 10° K./min.to a temperature of 200°-400° C., until the sample began to melt. AMettler DSC 20 was used and showed a peak at the point of fusion(melting).

Scanning Electron Microscope

A sample of membrane was sputtered with gold and measured in theScanning Electron Microscope under vacuum. The Scanning ElectronMicroscope used was a Jeol, JSM4500 commercially available from ContronElektronik, Germany.

Infrared Spectroscopy

The measurements were made with an infrared spectrometer IFS 66 of theBruker company. A sample having a diameter of 13 mm was clamped intoplace in the IFS Spectrometer. Light absorption was measured at 64 scanand a resolution of 4 cm⁻¹.

Break Elongation/Tear Resistance (Machine Direction)

The measurements were made on INSTRON (Instron Corporation SeriesIX--automatic material testing system 1.09) according to DIN standard 53888.

A sample membrane cut to the specifications of the DIN Standard wasclamped into place over a 50 mm length and elongated at 100 mm/min. at20° C. and 65% humidity. Elongation and resistance immediately beforebreakage were recorded by the machine.

Microporous Membrane from Electron-Irradiated PTFE EXAMPLE 1

A PTFE emulsion polymerisate made in accordance with EP0170382 waselectron irradiated at 500 Gy at room temperature to a melt flow indexcorresponding to that of PTFE micropowder TF9202 commercially availablefrom Hoechst AG of Germany and then tempered for 30 minutes at 200° C.One thousand twenty (1020) g of this irradiated PTFE emulsionpolymerisate was premixed for 10 minutes with 5784 g of the same PTFEemulsion polymerisate (made in accordance with EP0170382 and having amolecular weight greater than 5×10⁶) that was not irradiated. Alubricant, parrafin oil having a boiling point in the range 181° C. to212° C. (2.16 liters) was then added to the premixture and thecomponents were further mixed for twenty minutes.

The mixture was then pressed into a billet at 36 bars. Subsequentextrusion and calendering produced a PTFE tape measuring 0.15 mm inthickness. This tape was guided through a heating zone at 230° C. toremove the lubricant, longitudinally expanded at a rate of approximately500% per second at a ratio of 1:4 to form a membrane and sintered at370° C.

The properties of the resulting article (membrane) are shown in Table 1.

EXAMPLE 2

An emulsion polymerisate similar to that described in Example 1 waselectron irradiated under conditions identical to those described inExample 1. Four hundred eight (408) g of this irradiated emulsionpolymerisate was premixed for 10 minutes with 6396 g of PTFE emulsionpolymerisate, similar to that described in Example 1. A lubricant,paraffin oil having a boiling range of 181° C. to 212° C. (2.16 liters)was added to the premixture and the components were further mixed fortwenty minutes. The mixture was pressed into a billet at 36 bars.Subsequent paste extrusion and calendering produced a tape measuring0.155 mm in thickness. This tape was guided through a heating zone at230° C. to remove the lubricant, expanded at 1:4 in the longitudinaldirection at a rate of approximately 500% per second to form a membraneand sintered at 370° C.

The properties of this membrane are shown in Table 1.

EXAMPLE 3

An emulsion polymerisate, TF 2025, was obtained commercially fromHoechst AG of Germany. The emulsion polymerisate was electron irradiated(600 kGy) at room temperature and then tempered at 200° C. for 30minutes.

A PTFE emulsion polymerisate(molecular weight >5×10⁶) made in accordanceto EP0170382 (6396 g) was premixed with 408 g of the electron-irradiatedemulsion polymerisate (TF 2025) for 10 minutes. Paraffin oil having aboiling point in the range of 181° to 212° C. (2.22 liters) was added tothe components and mixed for 30 minutes. The mixture was pressed into abillet at 36 bars. Subsequent paste extrusion and calendering produced aPTFE tape measuring 0.152 mm in thickness. This tape was guided througha heating zone at 230° C. to remove the lubricant, expanded 1:4 in thelongitudinal direction at a rate of approximately 500% per second toform a membrane and sintered at 370° C.

The properties of this membrane are shown in Table 1.

EXAMPLE 4

Expanded PTFE made in accordance with U.S. Pat. No. 3,953,566 (GORE-TEX®film) was obtained and electron irradiated at room temperature at 300kGy. The GORE-TEX film did not contain any additives and was notprecleaned before irradiating. The electron irradiated PTFE was temperedat 200° C. for 30 minutes.

A PTFE emulsion polymerisate (molecular weight >5×10⁶) made inaccordance with EP0170382 (6396 g) was premixed with 408 g of theelectron-irradiated expanded PTFE (GORE-TEX® film) for 10 minutes.Paraffin oil having a boiling point in the range of 181° C. to 212° C.(2.22 liters) was added and the components were further mixed for 20minutes. The mixture was then pressed into a billet at 36 bars.Subsequent paste extrusion and calendering produced a PTFE-tapemeasuring 0.16 mm in thickness. This tape was guided through a heatingzone at 230° C. to remove the lubricant, expanded 1:4 in thelongitudinal direction at a rate of approximately 500% per second toform a membrane and sintered at 370° C.

The properties of this membrane are shown in Table 1.

FIG. 1 shows a photomicrograph taken at 2000× magnification of thesample membrane product.

FIG. 2 is an absorption scan of the Fourier transformation IRspectroscopy of the sample microporous PTFE structure.

Microporous Membrane With Radiation Degraded commercially available PTFEEXAMPLE 5

A PTFE emulsion polymerisate (molecular weight >5×10⁶) made inaccordance with EP0170382 (5784 g) was premixed with 1020 g of PTFEmicropowder MP 1200 commercially available from E. I. DuPont de Nemours,Inc. of Wilmington, Del. for 10 minutes. Paraffin oil having a boilingpoint in the range of 181° C. to 212° C. (2.07 liters) was added to thecomponents and further mixed for 20 minutes. The mixture was thenpressed into a billet at 36 bars. Subsequent paste extrusion andcalendering produced a PTFE-tape measuring 0,147 mm in thickness. Thistape was guided through a heating zone at 230° C. to remove thelubricant, expanded 1:4 in the longitudinal direction at a rate ofapproximately 500% per second to form a film and sintered at 370° C.

The properties of this membrane are shown in Table 1.

EXAMPLE 6

A PTFE emulsion polymerisate (molecular weight >5×10⁶) made inaccordance with EP0170382 (6396 g) was premixed with 408 g PTFEmicropowder MP 1200 commercially available from E. I. DuPont de Nemours,Inc. of Wilmington, Del. for 20 minutes. Paraffin oil having a boilingpoint in the range of 181° C to 212° C. (2.19 liters) was added to thecomponents and further mixed for 10 minutes. The mixture was pressedinto a billet at 14 bars. Subsequent paste extrusion and calenderingproduced a tape measuring 0.15 mm in thickness. This tape was guidedthrough a heating zone at 230° C. to remove the lubricant, expanded 1:4in the longitudinal direction at a rate of approximately 500% per secondto form a membrane and sintered at 370° C.

The properties of this membrane are shown in Table 1.

EXAMPLE 7

A PTFE emulsion polymerisate (molecular weight >5×10⁶) made inaccordance with EP0170382 (6396 g) was premixed with 408 g PTFEmicropowder MP 1300 commercially available from E. I. dupont de Nemours,Inc. of Wilmington, Del. for 10 minutes. Paraffin oil having a boilingpoint in the range of 181° C. to 212° C. (2.19 liters) was added to thecomponents and further mixed for 20 minutes. The mixture was thenpressed into a billet at 36 bars. Subsequent paste extrusion andcalendering produced a tape measuring 0.16 mm in thickness. This tapewas guided through a heating zone at 230° C. to remove the lubricant,expanded 1:4 in the longitudinal direction at a rate of approximately500% per second to form a membrane and sintered at 370° C.

The properties of this membrane are shown in Table 1.

Comparative Example A

A PTFE emulsion polymerisate (molecular weight >5×10⁶) made inaccordance with EP0170382 (27220 g) was mixed with 5.95 kg of paraffinoil having a boiling point in the range of 181° C. to 212° C. for 20minutes and pressed into a pellet at 36 bars. Subsequent paste extrusionand calendering produced a PTFE-tape measuring 0.19 mm in thickness.This tape was guided through a heating zone at 230° C. to remove thelubricant, expanded 1:4 in the longitudinal direction at a rate ofapproximately 500% per second to form a membrane and sintered at 370° C.

The properties of this membrane are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    PROPERTIES Of COMPOUND MEMBRANES                                                                 Bubble                                                          Particle size                                                                        Pore   Point     Melting                                                                            Break Tear                                       of Additives                                                                         Diameter                                                                             i-propanol                                                                          Gurley                                                                            Heat Elongation                                                                          Strength                              Example                                                                            in micrometer                                                                        in micrometer                                                                        in bar                                                                              in sec                                                                            in J/g                                                                             in %  in N/mm.sup.2                         __________________________________________________________________________    1    Agglomerate                                                                          0.8    0.5   6.5 36.7 94    38                                    2    Agglomerate                                                                          0.8    0.5   7   32.8 90    39                                    3    4      0.8    0.5   5.5 34.5 72    33                                    4    <10    1      0.6   9   33.0 77    38                                    5    3      0.8    0.5   5   36.9 122   38                                    6    3      0.7    0.6   7.5 33.5 149   61                                    7    15     0.8    0.5   11  41.3 80    36                                    A           2.0    0.2   3   31.5 61    21                                    __________________________________________________________________________

Microporous Unsintered Membrane With Electron-Irradiated PTFE EXAMPLE 8

An emulsion polymerisate TF2025 commercially available from Hoechst AGof Germany (106.6 g) was premixed for 10 minutes with 6.8 g ofelectron-irradiated (400 kGy) expanded PTFE made in accordance with U.S.Pat. No 3,953,566 that was not sintered. Paraffin oil having a boilingpoint in the range of 181° C. to 212° C. (34 mililiters) was added tothe components and further mixed for 20 minutes. The mixture was thenpressed into a billet in 15 sec. at 3.5 bars. The irradiation steps werethe same as those described in Example 1 except for the electronirradiation exposure which was 400 kGy for this example. Subsequentpaste extrusion at 176 bars and expansion (25:1) at a rate ofapproximately 1000% per second at 300° C. produced a film measuring 0.18mm in thickness and 13 mm in width. The results are summarized in Table2.

Comparative Example B

An emulsion polymerisate TF2025 commercially available from Hoechst AGof Germany (113.4 g) was premixed with 32.5 ml paraffin oil having aboiling range 181 ° C. to 212° C. for 20 minutes and pressed into abillet in 15 sec. at 3.5 bars. Subsequent paste extrusion at 181 barsand expansion (25:1) at a rate of approximately 1000% per second at 300°C. produced a film measuring 0.15 mm in thickness and 15 mm in width.The results are summarized in Table 2 and can be compared to the resultsof Example 8.

                  TABLE 2                                                         ______________________________________                                        Properties of Unsintered Compound Membranes                                          Particle                                                                      Size of    Melting    Break   Tear                                            Additives in                                                                             Heat       Elongation                                                                            Strength                                 Example                                                                              micrometers                                                                              J/g        %       .sub.-- N/mm.sup.2                       ______________________________________                                        8      <10        29.6       25      12.3                                     B        1        27.0       18      10.8                                     ______________________________________                                    

Microporous Biaxially Expanded Membrane With Electron-Irradiated PTFEEXAMPLE 9

Expanded PTFE (GORE-TEX® membrane) (544 g) made in accordance to U.S.Pat. No. 3,953,566 was electron irradiated at 400 kGy (particle sizeless than 10 micrometers) and dispersed in 2.22 liters of paraffin oilhaving boiling range from 181° C. to 212° C. for 10 minutes. A PTFEemulsion polymerisate (molecular weight greater than 5×10⁶) made inaccordance with EP0170382 (6260 g) was added. All components were mixedfor 20 minutes and pressed into a billet at 36 bars. Subsequent pasteextrusion and calendering produced a PTFE-tape measuring 0.2 mm inthickness. This tape was guided through a heating zone at 230° C. toremove the lubricant, expanded at 1:6 at a rate of approximately 200%per second in the cross direction and simultaneously expanded at 1:4 inthe longitudinal direction at approximately the same rate and sinteredat 370° C.

Comparative Example C

A PTFE emulsion polymerisate (molecular weight >5×10⁶) made inaccordance with EP0170382 was mixed with 5.90 kg of paraffin oil(boiling range 181° C. to 212° C.) for 20 minutes and pressed into apellet at 36 bars. Subsequent paste extrusion and calendering produced aPTFE-tape measuring 0.19 mm in thickness. This tape was guided through aheating zone at 230° C. to remove the lubricant, expanded at 1:6 at arate of approximately 200% per second in the cross direction andsimultaneously expanded at 1:4 in the longitudinal direction atapproximately the same rate and sintered at 370 ° C.

EXAMPLE 10

Samples made in accordance with Examples 1-9 and comparative Examples Aand C were subjected to impregnation and coating with additivesincluding polycations and silane coatings. Various tests were carriedout. The experiments confirmed that expanded compounds with irradiatedPTFE have an increased affinity to other polymers, coating systems andadhesive agents and have a reduced oleophobicity and hydrophilicity.Tables 3-6 summarize the additives used, amounts of additives and theresults.

                  TABLE 3                                                         ______________________________________                                        Loadability/Charge Reversal of membranes With                                 Polyelectrolytes                                                                                             Flow Potential                                           Flow       Polycation                                                                              [Zeta-potential]                               Example   Potential  Load*     After Loading                                  Membrane  in mV      in %      in mV                                          ______________________________________                                        A         -17.0      0.7       +20.0                                          4         -19.9      0.9       +17.2                                          5         -26.8      2.1       +17.9                                          ______________________________________                                         *A prewetted membrane was put in an aqueous 0.1 n polyallylamine solution     at 60° C. for 10 minutes and dried at 130° C.              

                  TABLE 4                                                         ______________________________________                                        Loadability/Reaction of the Membrane With Adhesive Agents                     Membrane    Silane***   Mass Increase in %                                    ______________________________________                                        A           A* 1100 (0.1 n)                                                                           0                                                     4           A* 1100 (0.1 n)                                                                           1.0                                                   5           A* 1100 (0.1 n)                                                                           1.3                                                   A           GF91** (0.1 n)                                                                            0                                                     4           GF91** (0.1 n)                                                                            1.3                                                   5           GF91** (0.1 n)                                                                            1.6                                                   ______________________________________                                         *A1100: H.sub.2 NCH.sub.2 CH.sub.2 NH(CH.sub.2)Si(OCH.sub.3).sub.3            **GF91: H.sub.2 N(CH.sub.2).sub.3 Si(OC.sub.2                                 ***A membrane was wetted with ipropanol and then put into a 0.1 n Shellso     solution (boiling point 60-95° C.) of the silane for 30 minutes.       The membrane was squeezed off, rinsed with water and dried at 130°     C.                                                                       

                  TABLE 5                                                         ______________________________________                                        Affinity/Adhesion of the Membranes in coating Systems                                               Coating                                                 Example               Thickness                                               Membrane Coating      Micrometers                                                                              Peel Test                                    ______________________________________                                        C        Polyetherurethane                                                                          10         Coating can                                                                   be torn off                                  9        Polyetherurethane                                                                          10         Membrane is                                                                   destroyed                                    ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                        Surface Characteristics of Membrane                                                    Flow Potential                                                                            Contact Angle                                                                             Contact Angle                                Example  of Membrane Against     Against                                      Membrane in mV       Water       CH.sub.2 I.sub.2                             ______________________________________                                        A        -17         128         116                                          (uniaxial)                                                                    C        -21.8       138         117                                          (biaxial)                                                                     1        -18.2       126         111                                          (uniaxial)                                                                    4        -19.9       121         111                                          (uniaxial)                                                                    9        -21.1       130         116                                          (biaxial)                                                                     5        -26.8       122         106                                          (uniaxial)                                                                    ______________________________________                                    

We claim:
 1. A porous polytetrafluoroethylene material comprising amixture of degraded polytetrafluoroethylene that had been subjected to aradiation degradation process wherein the degradedpolytetrafluoroethylene of the mixture has an average numericalmolecular weight of at most 10⁶, and a high-molecular emulsionpolymerisate of polytetrafluoroethylene wherein the emulsionpolymerizate of tetrafluoroethylene has a numerical average molecularweight between 2×10⁶ and 50×10⁶ wherein the mixture is further subjectedto a process comprising extrusion and expansion steps.
 2. A porouspolytetrafluoroethylene material as described in claim 1 which hasfurther undergone a sintering step.
 3. A porous polytetrafluoroethylenematerial as described in claim 1 wherein the degradedpolytetrafluoroethylene is degraded by means of electron rays.
 4. Aporous polytetrafluoroethylene material as described in claim 1 whereinan energy dose of 10 to 3000 kGy is used in the degradation process. 5.A porous polytetrafluoroethylene material as described in claim 1wherein the amount of degraded polytetrafluoroethylene is 1 to 50%,relative to the total mass.
 6. A porous polytetrafluoroethylene materialas described in claim 1 wherein the degraded polytetrafluoroethylene hasa particle size of 0.1 to 100 micrometers.